Concerned fans of Shaun of the Dead and Hot Fuzz can stop fretting. So can Nick Frost. Simon Pegg may have taken his talents as doleful comic underdog to star in Run, Fat Boy, Run, a glossy romcom directed by new best pal David Schwimmer, but Pegg is anxious to reassure everyone that he's still devoted to Frost, his old comedy partner. Pegg's second screen project with Schwimmer came about "purely by coincidence. It's not like he's replaced Nick. Because he never could." Pegg turns a little misty-eyed as he contemplates Frost, "my common-law wife".

Frost will be attending the premiere of Run, Fat Boy, Run. "He gets jealous of people," sighs Pegg. "He might punch David out." Frost and Pegg met when the former was a waiter in a Mexican restaurant in London. Frost obviously keeps him on his toes. "He does as well. Very much so," Pegg agrees quietly.

Pegg is less on his toes and more hobbling around on his heels after stepping into the pitiful training plimsolls of Dennis Doyle for Run, Fat Boy, Run. Pegg had just lost two stone to play rural supercop Nicholas Angel in Hot Fuzz and didn't have the time to put it back on, so he donned three prosthetic stomachs of differing sizes. Dennis is a commitment-phobic loser who has scarpered from his implausibly beautiful (and pregnant) true love Libby (Thandie Newton) at the altar. With Libby happily hooked up with a hunky American hedge-fund manager (Hank Azaria), Dennis sets about winning her back by running a marathon.

Given Pegg's comic tributes to the zombie genre with Shaun of the Dead and cop action movie with Hot Fuzz, you might expect a knowing retelling of something from the Four Weddings stable. But this, as Pegg makes clear, is a very different film. Frost and Edgar Wright, Pegg's usual collaborators, were not involved. Instead, Pegg was employed as a co-writer to "polish" the US script and make it work in a London setting. "Americans are slightly more comfortable with sentiment, and the original script was slightly more sentimental," says Pegg tactfully. "I made it a little bit more cynical, but not in a negative way."

From Dennis's eccentric gang of mates to east London's Columbia Road getting the Notting Hill treatment, Run, Fat Boy, Run looks more like a Working Title production than any Working Title film. Pegg agrees that the movie is "absolutely" pitched at the American market as well as British audiences, but defends the rather familiar starring role for the London Eye and the Gherkin. Apart from the marathon, he argues, the film is set in Hackney. "It's probably an even blend of postcard London and real London."

Then there is Nike. The film's marathon is called the Nike River Run, Dennis is given a box-fresh set of Nikes by his landlord and, at times, it appears that the capital itself is sponsored by the sports giant. Pegg sighs. "I know. I had a problem with that. The problem was we couldn't use the London Marathon, because they are tied up in another movie. So we had to invent this run. I cringed every time I came on set and there was all this corporate sponsorship everywhere. But sometimes, to make an omelette ..." He clears his throat with a wry grimace and struggles back on-message. "That was just the way it had to be, and of course we're very grateful to them for it."

Pegg's recipe for preventing a backlash against his popularity is to continue to play the underdog. "A backlash is fair enough if the quality drops, but if it's just for the sake of a backlash then that's just cynical," he reasons. "That's why you should always play losers because people love losers. The minute you threaten any kind of success, you're on the edge of being derided."

"Everyman" is a term often bandied in his direction. Average in height, looks and pallor, Pegg excels in the short-sleeved shirted uniforms of Britain's underpaid service sector. Is he insulted by this back-handed compliment? "Not really. It's weird. Me and Martin Freeman both get this, we're supposedly great 'Everymen', but I think it's just because we're fairly average," he says.

If Pegg is not an Everyman, then he is a posterboy for the kidult generation. Male fans who have flatmates and Xboxes well into their 30s will be relieved to hear that Run, Fat Boy, Run is not his ageing adolescent swansong. "Nick and I are writing a movie called Paul which we'll be shooting in America next year. We both play adolescent types in that," says Pegg, who married two years ago and lives quietly in north London. He may be more mature than his roles - he confesses to writing strident emails to the Guardian complaining about the preponderance of floral clues in its quick crossword - but remains fascinated by this audience that is growing up, or not, with him.

"Our parents' generation were encouraged to settle down and become adults a lot sooner, in their early 20s. Now we've suddenly got our whole 20s and even our 30s to arse around, and nobody is quite sure what to do with it, so all we do is regress back to when we were younger and find the things that we enjoyed doing then. There's this great shift towards nostalgia and looking back." He leans forward. "I'd like to get to the bottom of that. We're now being nostalgic about recent things. As soon as nostalgia catches up with the present, it will be Armageddon. When I see I Love Early 2010 in 2010, that's when we're in trouble."

Apart from his future project with Frost, Pegg is also eager to complete what he and his co-writing collaborator Edgar Wright call "the blood and ice-cream trilogy". Each film, he explains, "will feature a Cornetto in varying flavours. Shaun of the Dead has strawberry, Hot Fuzz has the original Cornetto flavour, and there will be a mint-choc chip in the third film. We don't quite know how or why yet, but it will happen. It's like Krzysztof Kieslowski but with confection. It's Three Colours: Cornetto." You can see the box set already: "With free ice cream."

After promoting Run, Fat Boy, Run, Pegg will return to his day job in his office in London - writing scripts, while sitting opposite Frost. "It's a tax on you. In order to have the autonomy that we have, you have to put in the hours," he explains. He's not complaining. "Now that's not at all boring. I love sitting opposite Nick Frost. I spend the day pissing myself laughing, but it's still a job. You have to go through that in order to do what you want to do. If I didn't write, there would be no Hot Fuzz, and my career would be considered less kind of, glittering. Don't put glittering - that sounds really fucking up myself." Pegg is blushing. "'Impressive' - that's the word, because there are levels of 'impressive'. Glittering just sounds like, 'Yeah, I'm great'".